Kinetics of the acid hydrolysis of nitrito-complexes. Part III. Nitritochromium(III) complexes

Abstract

Kinetic studies of the acid-catalysed hydrolysis of the nitrito-ligand from the complexes trans-[Cr(NH₃)₄(ONO)Cl]⁺, cis-[Cr(NH₃)₄(ONO)NCS]⁺, cis-[Cr(NH₃)₄(ONO)OH₂]²⁺, cis-[(NH₃)₅Cr(OH)Cr(NH₃)₄(ONO)]⁴⁺, and cis-[Cr(en)₂(ONO)(NCS)]⁺(en = ethylenediamine) have been carried out by the stopped-flow method. Below pH 1, a common rate law applies: –d [Complex]/dt=(k₁′[H⁺]+k₂′[H⁺]²)[Complex]. At 283·1 K and I= 1·0 mol I^–1(NaClO₄): 10²k₁′= 2·59 ± 0·07, 12·7 ± 1·6, 5·9 ± 0·2, 3·6 ± 0·2, 13·8 ± 0·4 l mol^–1 s^–1; and 10²k₂′= 6·27 ± 0·13, 86·1 ± 3·1, 12·9 ± 0·3, 2·9 ± 0·1, and 40·9 ± 0·5 I² mol^–1 s^–1 for these complexes respectively. For the complexes trans-[Cr(NH₃)₄(ONO)Cl]⁺ and cis-[Cr(NH₃)₄(ONO)NCS]⁺, activation parameters at 298·1 K are ΔH^‡(k₁′)= 20·3 ± 0·3 and 15·3 ± 0·1 kcal mol^–1, ΔS^‡(k₁′,)=+6 ± 1 and –8·4 ± 0·2 cal K^–1 mol^–1, ΔH^‡(k₂′)= 14·7 ± 0·2 and 11·9 ± 0·1 kcal mol^–1, and ΔS^‡(k₂′)=–12 ± 1 and –16·7 ± 0·2 cal K^–1 mol^–1 respectively. Neighbouring-group interaction between the ligands ONO^– and NCS^– is postulated to account for the low value of ΔH^‡(k₁′) in the complex cis-[Cr(NH₃)₄(ONO)NCS]⁺. All the reactions proceed with retention of configuration, and the results are consistent with a mechanism involving nitrogen–oxygen bond cleavage.